This invention relates generally to the measurement of the electrical resistance of a semiconductor sample and an apparatus for probing a semiconductor wafer. One commonly accepted technique for determining sheet resistivity is to use a four-point probe. Typically, four electrodes, or points, are placed in contact with the sample to be measured. A known electrical current is applied between two of the probes and conducted through the sample. The voltage between the other two probes is measured. The values of the current and voltage are used to calculate resistivity of the surface layer of the semiconductor sample according to known formulas.
Four-point probes are effective in measuring sheet resistivity in many cases. However, as the semiconductor industry becomes more technologically advanced, certain limitations of the four-point technique become apparent. Semiconductor devices are now being processed with very thin submicron diffusion or ion implantation layers on semiconductor wafers. Such structures are required for high speed and high frequency integrated circuits and transistors, as well as for higher efficiency solar cells.
One significant problem in using a four-point probe to characterized the layers is that for proper application of the known formula for calculating sheet resistivity, the current injected by the two current probes must be confined to the thin layer on the surface of the semiconductor. In reality, however, some of the injected current may not be confined to the thin surface layer. Instead, it leaks through to the underlying substrate. The leakage current conducted through the substrate cannot be detected by the four-point probe. Hence, the value of current used in the four-point probe formula produces an erroneous result for measured sheet resistivity.